fixed one indexing bug in the hessian computation; added various helper methods for the derivatives of the strains and fd approximations for them

[[Imported from SVN: r1501]]

[[Imported from SVN: r1472]]

[[Imported from SVN: r1220]]

bugfix: the explicit expression for the Darboux vector given by Dichmann is already in local coordinates

[[Imported from SVN: r1105]]

[[Imported from SVN: r1087]]

[[Imported from SVN: r1075]]

[[Imported from SVN: r1040]]

[[Imported from SVN: r850]]

- Factorize derivatives of directors in separate method
- sign fixes

[[Imported from SVN: r845]]

[[Imported from SVN: r840]]

[[Imported from SVN: r834]]

[[Imported from SVN: r817]]

[[Imported from SVN: r742]]

[[Imported from SVN: r740]]

[[Imported from SVN: r597]]

[[Imported from SVN: r588]]

[[Imported from SVN: r482]]

[[Imported from SVN: r410]]